1. Field of the Invention
The present invention relates to spool valves, and more particularly to such valves that have an integral check valve for controlling fluid flow through the valve.
2. Description of the Related Art
A wide variety of machines have moveable members that are operated by an hydraulic actuator. For example an internal combustion engine has a camshaft which is mechanically coupled to rotate with the crankshaft and which opens and closed cylinder intake and exhaust valves. Traditionally the camshaft timing was fixed at a setting that produced the best operation at all engine operating speeds. However, it has been recognized that engine performance can be improved if the valve timing varies as a function of engine speed, engine load, and other factors. Thus a hydraulic actuator is being used on some engines to vary the coupling relationship of the camshaft to the crankshaft and a solenoid operated valve is employed to control the application of pressurized fluid to operate the hydraulic actuator.
Hydraulic systems for controlling engine operation included check valves that allowed fluid to flow in only one direction. For example, a separate check valve connected in a conduit coupled to an inlet or an outlet of solenoid operated valve permitted fluid to flow only to or from that latter valve. Providing the solenoid operated valve and check valve as separate components increased the number of parts to connect together in the hydraulic system. Therefore, it is desirable to integrate the check valve into the solenoid operated valve to simplify connection of the hydraulic system.